By default, the configuration uses SQLite. If you’re new to databases, or
you’re just interested in trying Django, this is the easiest choice. SQLite is
included in Python, so you won’t need to install anything else to support your
database. When starting your first real project, however, you may want to use a
more scalable database like PostgreSQL, to avoid database-switching headaches
down the road.

If you wish to use another database, install the appropriate database
bindings and change the following keys in the
DATABASES'default' item to match your database connection
settings:

ENGINE – Either
'django.db.backends.sqlite3',
'django.db.backends.postgresql',
'django.db.backends.mysql', or
'django.db.backends.oracle'. Other backends are also available.

NAME – The name of your database. If you’re using SQLite, the
database will be a file on your computer; in that case, NAME
should be the full absolute path, including filename, of that file. The
default value, BASE_DIR/'db.sqlite3', will store the file in your
project directory.

If you are not using SQLite as your database, additional settings such as
USER, PASSWORD, and HOST must be added.
For more details, see the reference documentation for DATABASES.

For databases other than SQLite

If you’re using a database besides SQLite, make sure you’ve created a
database by this point. Do that with “CREATEDATABASEdatabase_name;”
within your database’s interactive prompt.

Also make sure that the database user provided in mysite/settings.py
has “create database” privileges. This allows automatic creation of a
test database which will be needed in a later
tutorial.

If you’re using SQLite, you don’t need to create anything beforehand - the
database file will be created automatically when it is needed.

While you’re editing mysite/settings.py, set TIME_ZONE to
your time zone.

Also, note the INSTALLED_APPS setting at the top of the file. That
holds the names of all Django applications that are activated in this Django
instance. Apps can be used in multiple projects, and you can package and
distribute them for use by others in their projects.

By default, INSTALLED_APPS contains the following apps, all of which
come with Django:

These applications are included by default as a convenience for the common case.

Some of these applications make use of at least one database table, though,
so we need to create the tables in the database before we can use them. To do
that, run the following command:

$ python manage.py migrate

The migrate command looks at the INSTALLED_APPS setting
and creates any necessary database tables according to the database settings
in your mysite/settings.py file and the database migrations shipped
with the app (we’ll cover those later). You’ll see a message for each
migration it applies. If you’re interested, run the command-line client for your
database and type \dt (PostgreSQL), SHOWTABLES; (MariaDB, MySQL),
.schema (SQLite), or SELECTTABLE_NAMEFROMUSER_TABLES; (Oracle) to
display the tables Django created.

For the minimalists

Like we said above, the default applications are included for the common
case, but not everybody needs them. If you don’t need any or all of them,
feel free to comment-out or delete the appropriate line(s) from
INSTALLED_APPS before running migrate. The
migrate command will only run migrations for apps in
INSTALLED_APPS.

A model is the single, definitive source of truth about your data. It contains
the essential fields and behaviors of the data you’re storing. Django follows
the DRY Principle. The goal is to define your data model in one
place and automatically derive things from it.

This includes the migrations - unlike in Ruby On Rails, for example, migrations
are entirely derived from your models file, and are essentially a
history that Django can roll through to update your database schema to
match your current models.

In our poll app, we’ll create two models: Question and Choice. A
Question has a question and a publication date. A Choice has two
fields: the text of the choice and a vote tally. Each Choice is associated
with a Question.

These concepts are represented by Python classes. Edit the
polls/models.py file so it looks like this:

Here, each model is represented by a class that subclasses
django.db.models.Model. Each model has a number of class variables,
each of which represents a database field in the model.

Each field is represented by an instance of a Field
class – e.g., CharField for character fields and
DateTimeField for datetimes. This tells Django what
type of data each field holds.

The name of each Field instance (e.g.
question_text or pub_date) is the field’s name, in machine-friendly
format. You’ll use this value in your Python code, and your database will use
it as the column name.

You can use an optional first positional argument to a
Field to designate a human-readable name. That’s used
in a couple of introspective parts of Django, and it doubles as documentation.
If this field isn’t provided, Django will use the machine-readable name. In this
example, we’ve only defined a human-readable name for Question.pub_date.
For all other fields in this model, the field’s machine-readable name will
suffice as its human-readable name.

Some Field classes have required arguments.
CharField, for example, requires that you give it a
max_length. That’s used not only in the
database schema, but in validation, as we’ll soon see.

A Field can also have various optional arguments; in
this case, we’ve set the default value of
votes to 0.

Finally, note a relationship is defined, using
ForeignKey. That tells Django each Choice is
related to a single Question. Django supports all the common database
relationships: many-to-one, many-to-many, and one-to-one.

But first we need to tell our project that the polls app is installed.

Philosophy

Django apps are “pluggable”: You can use an app in multiple projects, and
you can distribute apps, because they don’t have to be tied to a given
Django installation.

To include the app in our project, we need to add a reference to its
configuration class in the INSTALLED_APPS setting. The
PollsConfig class is in the polls/apps.py file, so its dotted path
is 'polls.apps.PollsConfig'. Edit the mysite/settings.py file and
add that dotted path to the INSTALLED_APPS setting. It’ll look like
this:

By running makemigrations, you’re telling Django that you’ve made
some changes to your models (in this case, you’ve made new ones) and that
you’d like the changes to be stored as a migration.

Migrations are how Django stores changes to your models (and thus your
database schema) - they’re files on disk. You can read the migration for your
new model if you like; it’s the file polls/migrations/0001_initial.py.
Don’t worry, you’re not expected to read them every time Django makes one, but
they’re designed to be human-editable in case you want to manually tweak how
Django changes things.

There’s a command that will run the migrations for you and manage your database
schema automatically - that’s called migrate, and we’ll come to it in a
moment - but first, let’s see what SQL that migration would run. The
sqlmigrate command takes migration names and returns their SQL:

$ python manage.py sqlmigrate polls 0001

You should see something similar to the following (we’ve reformatted it for
readability):

By convention, Django appends "_id" to the foreign key field name.
(Yes, you can override this, as well.)

The foreign key relationship is made explicit by a FOREIGNKEY
constraint. Don’t worry about the DEFERRABLE parts; it’s telling
PostgreSQL to not enforce the foreign key until the end of the transaction.

It’s tailored to the database you’re using, so database-specific field types
such as auto_increment (MySQL), serial (PostgreSQL), or integerprimarykeyautoincrement (SQLite) are handled for you automatically. Same
goes for the quoting of field names – e.g., using double quotes or
single quotes.

The sqlmigrate command doesn’t actually run the migration on your
database - instead, it prints it to the screen so that you can see what SQL
Django thinks is required. It’s useful for checking what Django is going to
do or if you have database administrators who require SQL scripts for
changes.

If you’re interested, you can also run
pythonmanage.pycheck; this checks for any problems in
your project without making migrations or touching the database.

The migrate command takes all the migrations that haven’t been
applied (Django tracks which ones are applied using a special table in your
database called django_migrations) and runs them against your database -
essentially, synchronizing the changes you made to your models with the schema
in the database.

Migrations are very powerful and let you change your models over time, as you
develop your project, without the need to delete your database or tables and
make new ones - it specializes in upgrading your database live, without
losing data. We’ll cover them in more depth in a later part of the tutorial,
but for now, remember the three-step guide to making model changes:

The reason that there are separate commands to make and apply migrations is
because you’ll commit migrations to your version control system and ship them
with your app; they not only make your development easier, they’re also
usable by other developers and in production.

>>> frompolls.modelsimportChoice,Question# Import the model classes we just wrote.# No questions are in the system yet.>>> Question.objects.all()<QuerySet []># Create a new Question.# Support for time zones is enabled in the default settings file, so# Django expects a datetime with tzinfo for pub_date. Use timezone.now()# instead of datetime.datetime.now() and it will do the right thing.>>> fromdjango.utilsimporttimezone>>> q=Question(question_text="What's new?",pub_date=timezone.now())# Save the object into the database. You have to call save() explicitly.>>> q.save()# Now it has an ID.>>> q.id1# Access model field values via Python attributes.>>> q.question_text"What's new?">>> q.pub_datedatetime.datetime(2012, 2, 26, 13, 0, 0, 775217, tzinfo=<UTC>)# Change values by changing the attributes, then calling save().>>> q.question_text="What's up?">>> q.save()# objects.all() displays all the questions in the database.>>> Question.objects.all()<QuerySet [<Question: Question object (1)>]>

Wait a minute. <Question:Questionobject(1)> isn’t a helpful
representation of this object. Let’s fix that by editing the Question model
(in the polls/models.py file) and adding a
__str__() method to both Question and
Choice:

It’s important to add __str__() methods to your
models, not only for your own convenience when dealing with the interactive
prompt, but also because objects’ representations are used throughout Django’s
automatically-generated admin.

Note the addition of importdatetime and fromdjango.utilsimporttimezone, to reference Python’s standard datetime module and Django’s
time-zone-related utilities in django.utils.timezone, respectively. If
you aren’t familiar with time zone handling in Python, you can learn more in
the time zone support docs.

Save these changes and start a new Python interactive shell by running
pythonmanage.pyshell again:

>>> frompolls.modelsimportChoice,Question# Make sure our __str__() addition worked.>>> Question.objects.all()<QuerySet [<Question: What's up?>]># Django provides a rich database lookup API that's entirely driven by# keyword arguments.>>> Question.objects.filter(id=1)<QuerySet [<Question: What's up?>]>>>> Question.objects.filter(question_text__startswith='What')<QuerySet [<Question: What's up?>]># Get the question that was published this year.>>> fromdjango.utilsimporttimezone>>> current_year=timezone.now().year>>> Question.objects.get(pub_date__year=current_year)<Question: What's up?># Request an ID that doesn't exist, this will raise an exception.>>> Question.objects.get(id=2)Traceback (most recent call last):...DoesNotExist: Question matching query does not exist.# Lookup by a primary key is the most common case, so Django provides a# shortcut for primary-key exact lookups.# The following is identical to Question.objects.get(id=1).>>> Question.objects.get(pk=1)<Question: What's up?># Make sure our custom method worked.>>> q=Question.objects.get(pk=1)>>> q.was_published_recently()True# Give the Question a couple of Choices. The create call constructs a new# Choice object, does the INSERT statement, adds the choice to the set# of available choices and returns the new Choice object. Django creates# a set to hold the "other side" of a ForeignKey relation# (e.g. a question's choice) which can be accessed via the API.>>> q=Question.objects.get(pk=1)# Display any choices from the related object set -- none so far.>>> q.choice_set.all()<QuerySet []># Create three choices.>>> q.choice_set.create(choice_text='Not much',votes=0)<Choice: Not much>>>> q.choice_set.create(choice_text='The sky',votes=0)<Choice: The sky>>>> c=q.choice_set.create(choice_text='Just hacking again',votes=0)# Choice objects have API access to their related Question objects.>>> c.question<Question: What's up?># And vice versa: Question objects get access to Choice objects.>>> q.choice_set.all()<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>>>> q.choice_set.count()3# The API automatically follows relationships as far as you need.# Use double underscores to separate relationships.# This works as many levels deep as you want; there's no limit.# Find all Choices for any question whose pub_date is in this year# (reusing the 'current_year' variable we created above).>>> Choice.objects.filter(question__pub_date__year=current_year)<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]># Let's delete one of the choices. Use delete() for that.>>> c=q.choice_set.filter(choice_text__startswith='Just hacking')>>> c.delete()

Generating admin sites for your staff or clients to add, change, and delete
content is tedious work that doesn’t require much creativity. For that
reason, Django entirely automates creation of admin interfaces for models.

Django was written in a newsroom environment, with a very clear separation
between “content publishers” and the “public” site. Site managers use the
system to add news stories, events, sports scores, etc., and that content is
displayed on the public site. Django solves the problem of creating a
unified interface for site administrators to edit content.

The admin isn’t intended to be used by site visitors. It’s for site
managers.

Now that we’ve registered Question, Django knows that it should be displayed on
the admin index page:

Click “Questions”. Now you’re at the “change list” page for questions. This page
displays all the questions in the database and lets you choose one to change it.
There’s the “What’s up?” question we created earlier:

Click the “What’s up?” question to edit it:

Things to note here:

The form is automatically generated from the Question model.

The different model field types (DateTimeField,
CharField) correspond to the appropriate HTML
input widget. Each type of field knows how to display itself in the Django
admin.

Each DateTimeField gets free JavaScript
shortcuts. Dates get a “Today” shortcut and calendar popup, and times get
a “Now” shortcut and a convenient popup that lists commonly entered times.

The bottom part of the page gives you a couple of options:

Save – Saves changes and returns to the change-list page for this type of
object.

Save and continue editing – Saves changes and reloads the admin page for
this object.

Save and add another – Saves changes and loads a new, blank form for this
type of object.

Delete – Displays a delete confirmation page.

If the value of “Date published” doesn’t match the time when you created the
question in Tutorial 1, it probably
means you forgot to set the correct value for the TIME_ZONE setting.
Change it, reload the page and check that the correct value appears.

Change the “Date published” by clicking the “Today” and “Now” shortcuts. Then
click “Save and continue editing.” Then click “History” in the upper right.
You’ll see a page listing all changes made to this object via the Django admin,
with the timestamp and username of the person who made the change:

When you’re comfortable with the models API and have familiarized yourself with
the admin site, read part 3 of this tutorial to learn
about how to add more views to our polls app.